1. Field of the Invention
The present invention relates to a lubricant composition for hot plastic working containing graphite. More particularly, the present invention relates to a graphite-containing lubricant composition for hot plastic working, having an excellent inhibiting effect against carburizing phenomenon, which often occurs when the graphite-containing lubricant is applied to a metal material.
2. Description of Related Art
When a metal material is subjected to a hot plastic working, for example, hot rolling, extrusion or forging, a lubricant is used to decrease friction, for example between a die for forging and the metal material to be worked, to prevent galling or seizing and to render the plastic working easy.
Generally, the lubricant is classified into two groups, namely a graphite-type lubricant and a nongraphite-type lubricant.
The graphite-type lubricant comprises graphite as a principal component. The nongraphite-type lubricant comprises, as a principal component, boron nitride, silicate or calcium carbonate. Each of the graphite type and nongraphite type lubricants contains a binder consisting of an inorganic compound such as boric acid or an organic polymeric compound such as a water-soluble resin, and is applied in the form of an aqueous liquid to the inside or outside surface of the die by way of, for example, spraying.
As regards the working environment, the nongraphite-type lubricant is preferable over the black graphite-type lubricant, and thus has recently been widely studied. However, the nongraphite-type lubricant is inferior to the graphite-type lubricant with respect to the lubricity. Therefore, the graphite-type lubricant is still widely employed for various uses.
Nevertheless, the graphite type lubricant has a disadvantageous in that since the graphite-type lubricant comprises graphite as an essential component that causes the resultant lubricant to exhibit an excellent lubricating effect, the occurrence of an undesirable carburizing phenomenon on the worked metal material is unavoidable due to the graphite.
This carburizing phenomenon greatly affects the worked metal material, especially when the metal material has a high upper limit of solid solubility of carbon at hot working temperatures, for example, a high alloy steel material is hot worked in the austenite state thereof. Namely, the carburizing phenomenon sometimes promotes undesirable hardening or cracking of the metal material, and thus can promote poor results in the subsequent forming process or can promote lowered product durability.
In response to the above-mentioned problem, new types of lubricants free from graphite are disclosed in JP-A-61-223,096 and JP-A-64-16,894. Nevertheless, a nongraphite-type lubricant comparable in lubricity to a graphite-type lubricant has not yet been developed.